This invention relates to an optical head which is employable in an optical disk device, an optical video-disk device, a compact-disk player and so on.
FIG. 1 shows a plane view showing a construction of a conventional optical head which is emplyable in an optical disk device. In the drawing, numeral 1 indicates a bobbin serving as a lens holder constructed to mount an objective lens 2 thereon. The bobbin 1 is arranged to be movable along a direction indicated by an arrow "A" as well as in a direction indicated by a pair of symbols "B1" and "B2" which is orthogonal to the "A" direction, i.e., perpendicular to a surface of a sheet on which the drawing is shown. Numeral 3 indicates a focusing coil wound around the outer surface of the bobbin 1, numerals 4A through 4D indicate tracking coils mounted on the focusing coil 3. Further, numerals 5, 5 indicates a wire suspension mechanism for supporting the bobbin 1. One pair of terminals of the wire suspension mechanism 5 is connected to an outer surface of the bobbin 1, while the other pair of terminals thereof is connected to a fixing member 6 which is fixed within a device in which the optical head is to be provided. Therefore, the bobbin 1 is supported by the wire suspension mechanism 5 and is moved in the "A" direction as well as "B1-B2" direction in accordance with deformation of wires comprising the wire suspension mechanism 5.
Numerals 7A through 7D respectively indicate permanent magnets mounted on fixing members 12A through 12D for generating a magnetic field. Thus, the focusing coil 3 and the tracking coils 4A through 4D are located in the magnetic field generated by the permanent magnets 7A through 7D. Numeral 8 indicates a reflecting plate for reflecting light transmitted from light transmitting elements included in a pair of optical sensors 9A and 9B, described later. The reflecting plate 8 is mounted on the bobbin 1, as shown in FIG. 1, on the opposite side surface from the surface on which the suspension mechanism 5 is connected. The reflecting plate 8 is arranged, for example, in such a manner that a surface of a metal, glass and so forth having been ground in advance, is evaporated by thin metal membrane. Further, a pair of optical sensors 9A and 9B are provided opposite the reflecting plate 8. Numeral 10 indicates a differential operational amplifier for outputting a signal in accordance with the difference between the outputs of the optical sensors 9A and 9B.
FIG. 2 shows an enlarged view of one of the optical sensors 9B. As shown in FIG. 2, the optical sensor 9B comprises a package 9B3 for accommodating a light transmitting element 9B1 such as a LED (Light Emitting Diode) and a light receiving element 9B2 such as a photodiode. Further, as shown in FIG. 3, the optical sensors 9A and 9B are arranged in a row along the "A" direction, and opposed to the reflecting plate 8. The optical sensor 9A like the optical sensor 9B, comprises a package 9A3 accommodating a light emitting element 9A1 and a light receiving element 9A2. As shown in FIG. 3, each of the pairs of the light transmitting and receiving elements, 9A1 and 9B1, 9A2 and 9B2, are separately located from each other in the "B1-B2" direction. The optical sensors 9A and 9B are mounted on a fixing unit, not shown, fixed within the device in which the optical head is to be provided. The light receiving elements 9A2 and 9B2 are arranged to be located on the outside of the reflecting plate 8 in the "A" direction, while the light transmitting elements 9A1 and 9B1 are located on the inside of the reflecting plate 8 as shown in FIG. 3. In other words, the light transmitted from the light transmitting elements 9A1 and 9B1 are respectively reflected at the both side edge portions of the reflecting plate 8 and received by the light receiving elements 9A2 and 9B2 as shown in FIG. 1.
In the above structured conventional optical head, when a signal caused by a moving error of the optical head along a focusing direction, i.e., in the "B1-B2" direction, is supplied to the focusing coil 3, moving force is generated along the "B1-B2" direction in accord with Fleming's left hand rule, since the focusing coil 3 is located in the magnetic field generated by the permanent magnets 7A through 7D. Accordingly, the wires of the suspension mechanism 5 are deformed, and then, the optical head can be moved along the "B1-B2" direction. On the other hand, when a signal caused by a moving error of the optical head along a tracking direction, i.e., in the "A" direction, is supplied to the tracking coils 4A through 4D, moving force is generated along the "A" direction in accord with the Fleming's left hand rule, and then, the optical head can be moved along the "A" direction. When the optical head is moved upwardly in the drawing of FIG. 1, the amount of light reflected by the reflecting plate 8 transmitted from the light transmitting element 9A1 is increased. Thus, the amount of light received by the light receiving element 9A2 is increased. In this case, received by the light receiving element 9B2 is decreased. On the contrary, when the optical head is moved downward in the drawing, the amount of light received by the light receiving element 9B2 is increased, and the amount of light received by the light receiving element 9A2 is decreased. Accordingly, by detecting the difference between the outputs "A" and "B", from these optical sensors which respectively correspond to the amounts of the received light, it becomes possible to determine the position of the optical head along the "A" direction, i.e., the tracking direction.
In the above conventional optical head, as shown in FIG. 3, since the pairs of elements 9A1, 9A2 and 9B1, 9B2 are deviated from each other in the "B1-B2" direction, the outputs "A" and "B" from the optical sensors 9A and 9B are erroneously varied when the optical head is moved in the "B1-B2" direction. In other words, when the optical head is moved upwardly in FIG. 3, the amount of light received by the light receiving element 9A2 is decreased. On the contrary, when the optical head is moved downwardly, the amount of light to be received by the light receiving element 9B2 is decreased. The above variation of light are erroneously detected as movement of the optical head in the "A" direction.
In order to solve the above problem, it can be arranged in such a manner that length of the reflecting plate 8 in the focusing direction is longer. However, this arrangement makes the total volume of the device larger, and the production cost higher.